1. Field of the Invention
The invention relates to an adjustable-length compression spring comprising a casing with a central longitudinal axis, the casing being filled with free-flowing pressure fluid; a guide and seal unit which closes the casing at a first end; a piston rod which, through the guide and seal unit, is extended from, and sealed towards, the first end of the casing, having an outer end; a piston which is joined to the piston rod and sealingly guided in the casing; a first sectional casing chamber which is unilaterally defined by the piston and filled with pressure fluid; a second sectional casing chamber which is connectable to the first sectional casing chamber and filled with pressure fluid; and an operating valve for connection to each other of the sectional casing chambers by an actuation and overflow assembly, the valve comprising a valve pin which, from outside the casing, is movable into a tripped position, in which the valve pin enables the actuation and overflow assembly to be released, and into a shut-off position, in which the valve pin shuts off the actuation and overflow assembly. The invention further relates to a seat comprising a compression spring of this type.
2. Background Art
A blockable compression spring of the generic type is known from EP 1 288 525 A2. Two valves are arranged in series in the actuation and over-flow assembly between the two sectional casing chambers, one of them being the operating valve that includes the valve pin as mentioned above. The second valve is a relay valve that blocks the entire actuation and over-flow assembly independently of the respective position of the operating valve. When the operating valve is open, the relay valve can be transferred by pressure actuation into a position of release with pressure fluid flowing freely between the two sectional casing chambers. This known gas spring is used in adjustable-backrest seats of vehicles. When a backrest is tilted backwards and an upright position is needed, a user will start by actuating the operating valve. Without a user's back pressing on the backrest, the relay valve stays in the position of blocking so that the backrest is kept in the originally tilted position even when the operating valve is tripped. Only when the user leans back sufficiently for generation, by the backrest, of control pressure that transfers the relay valve into a position of release, the operating valve and the relay valve are open, releasing the actuation and overflow assembly. Only then the backrest will conform to a more upright position of a user's back when he straightens up. This idea of transferring the backrest into a more upright position may prevent the compression spring, after actuation of the operating valve, from extending inadvertently and hitting against a user's back, but it is complicated and, above all, hardly intuitive. When the operating valve has been actuated, the user, though wishing a more upright position, must start by leaning back.